1. Field of the Invention
The present invention relates to a method for aligning a cylindrical laser optical resonator and more particularly to a method for aligning a high extraction decentered annular ring resonator (HEXDARR).
2. Description of the Prior Art
Various types of lasers are known in the art. For example, chemical lasers are known. Examples of such chemical lasers are disclosed in U.S. Pat. Nos. 3,575,669; 3,992,685; 4,237,429; 4,514,698; 5,099,492; and 5,624,564, hereby incorporated by reference. In order to provide relatively high power lasers in a relatively compact configuration, lasers utilizing a cylindrical gain generator and an annular ring resonator have been developed. In such lasers, the cylindrical gain generator is disposed within an annular ring resonator. In such a configuration, the gain medium flows radially outwardly from the cylindrical gain generator into an annular gain volume of the cylindrical resonator. In order to extract the maximum amount of power from the annular gain volume, high extraction annular ring resonators have been developed, for example, as disclosed in U.S. Pat. Nos. 4,598,408 and 4,744,090, and in the aforementioned pending ""918 patent application, assigned to the same assignee as the present invention hereby incorporated by reference. Examples of other annular ring resonators are also disclosed in U.S. Pat. Nos. 4,606,036 and 4,516,214. The annular ring resonators, for example, as disclosed in the ""408 patent, include a conical rear reflector, a waxicon, a reflaxicon, a scraper mirror with a decentered aperture and a plurality of flat beam steering mirrors forming a compact leg of the resonator. The decentered aperture in the scraper mirror allows a circular beam of light to be transmitted therethrough and reflected in the compact leg of the waxicon. The waxicon expands the circular beam into an annular light beam which makes a first pass through the annular gain volume. The annular beam is reflected by the conical rear reflector and thus makes a second pass through the annular gain volume. The annular beam reflected from the rear reflector is reflected to the reflaxicon, where the beam is compacted and a portion thereof reflected through the decentered aperture of the scraper mirror as feedback with the balance of the beam outcoupled as an output beam.
While such annular ring resonators provide relatively high gain, such resonators are not without disadvantages. For example, the optical elements behave quite differently from the ordinary optics used for conventional laser resonator designs. In general, tilt or decentration of these conical mirrors will result in a combination of phase tilt and other high-order phase aberrations in the output beam. Therefore, the resonator alignment becomes more complicated when the characteristics of the output beam are used to monitor the misalignment status of the resonator. Several alignment techniques have been developed for HEXDARR in the past few years. Earlier techniques were based on minimizing the observed near-field wave front error (WFE). Recently, an alignment method based on far-field patterns has been proposed (previously referenced Ser. No. 09/231,918) and used in high-energy laser tests. However these techniques do not provide a definite answer to the misalignment status of the resonator. In general, they are iterative procedures involving trials and errors. Furthermore, most techniques need human interface for pattern recognition and human""s judgement for adjusting the optics. These techniques may be working satisfactorially for the current ground tests; however, for a space-based laser system, an auto-alignment technique with a quick and accurate assessment of the misalignment status is very important.
It is an object of the present invention to solve various problems in the prior art.
It is yet another object of the present invention to provide a simplified method for aligning the optical surfaces within a cylindrical ring resonator.
Briefly, the present invention relates to a method for aligning the optical surfaces of the high extraction annular ring resonator which includes a scraper mirror with a decentered aperture. A probe beam from an external laser is directed into the resonator cavity through the decentered aperture in the scraper mirror. A visual and quantitative assessment of the amount of compact leg tilt and rear cone decentration is made, and the optical surfaces of the resonator are then repositioned into alignment based upon the results of the assessment.